The quality of photographic materials is often measured in terms of the materials' speed/grain performance. That is, materials which exhibit high sensitivity or speed, as well as low granularity, are desired for being capable of delivering to the consumer the highest quality images.
Sensitivity may be improved in many ways, such as by adding chemical sensitizers to an emulsion during its formation, or by modifying the morphology and/or halide content or distribution of an emulsion's grains. The average size of the grains contained within an emulsion is also significantly related to sensitivity. The larger the grains, the greater the number of incident photons per grain at a given exposure, and thus the higher the probability that a latent image center will be formed. Increasing an emulsion's sensitivity solely by increasing its grain size, however, has attendant disadvantages, one of which is to increase the emulsion's granularity relative to a smaller grain emulsion exhibiting equivalent final image density. Increased granularity, in turn, impairs image quality, especially where magnification of the image is required, such as in enlarged prints or transparencies.
In black and white films in particular, control over granularity is desired. In these films, it is typical to coat relatively high levels of silver halide grains in an attempt to maximize the number of image centers and to provide the lowest granularity at a given sensitivity. By coating such high levels of silver halide, however, development can not be carried to completion because it would result in an image having unacceptably high contrast. This problem can be eliminated by a technique called partial grain development, which is the process of carrying out development for a controlled period of time and arresting it at an appropriate point prior to its completion when the desired contrast has been obtained. Precise control over contrast during partial grain development can be achieved by modifying the activity of the developer utilized in the development process. Alternatively, and generally preferably as it does not require any modification of the development process, control over contrast can be achieved by incorporating into the photographic emulsion a compound capable of restraining development of the exposed silver halide, such compound typically being referred to in the art as a development inhibitor or, simply, an inhibitor.
One of the characteristics that development inhibitors in black and white photographic elements must exhibit is that they restrain only the growth rate of grain development and not the initiation of grain growth. Otherwise, the number of image centers in the emulsion would be decreased by the development inhibitor and any speed/grain advantage resulting from the utilization of high levels of silver halide grains would be lost. To this end, the photographic industry has developed and utilized various release compounds which are capable of releasing development inhibitors and, for that matter, other photographically active moieties, after the initiation of grain growth. Such compounds have typically been comprised of blocking groups which release development inhibitors through a cross-oxidation reaction. Long, U.S. Pat. No. 4,948,714, is exemplary of this art, and it specifically describes compounds which imagewise release 1-aryltetrazole-5-thiol development inhibitors through a cross-oxidation reaction in the presence of a black and white developing solution.
The utility of compounds which release development inhibitors via a cross-oxidation reaction in black and white developing solutions is limited by the high sulfite levels typically present in such solutions. Sulfite acts as a scavenger of oxidized developer and thus interferes with the ability of the release compound to be oxidized and to release its development inhibitor. Therefore, alternative release compounds for the restraint of black and white processing have been sought.
In co-pending U.S. Ser. Nos. 08/250,148, 08/250,748 and 08/250,189 release compounds have been disclosed which react with nucleophiles contained in processing baths to release photographically active moieties in a non-imagewise manner. The release compounds are asserted to be of particular use in color reversal photographic elements where control over push (i.e., extended) processing is desired. They comprise a blocking group from which a photographically active moiety is released, the blocking group comprising both a solubilizing groups and a ballasting group. The solubilizing group enables release of the photographically active moiety from the release compound.
Although such compounds would likely release photographically active moieties in black and white films during development, they would be impractical as they would require substantial amounts of strong, environmentally harmful organic solvents in order to be incorporated into an emulsion. In color films, such as the reversal films of U.S. Ser. Nos. 08/250,148, 08/250,748 and 08/250,189, this problem is eliminated since the compounds can be incorporated into coupler dispersions or other solvents utilized in the coupler containing layers, such as diethyl lauramide. In black and white film, however, couplers and hence coupler solvents are not utilized, and thus alternative means for incorporating release compounds are needed.
One organic solvent that is commonly used in the preparation of most black and white films is methanol, and it has been used as a vehicle by which to add various hydrophobic addenda to gelatin containing emulsion layers. The present inventors have found, however, that the release compounds of U.S. Ser. Nos. 08/250,148, 08/250,748 and 08/250,189 are only marginally soluble in methanol and are thus relatively impractical for application to most black and white films where the manufacture of such films is desired to be achieved without the use of substantial amounts of strong environmentally harmful organic solvents. Applicants therefore sought to identify a class of release compounds at least as effective as those described in U.S. Ser. Nos. 08/250,148, 08/250,748 and 08/250,189, and capable of being used in modern black and white films where environmental considerations are of particular concern.